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  并发容器
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  <h1 id="并发容器"><a href="#并发容器" class="headerlink" title="并发容器"></a>并发容器</h1><hr>
<h2 id="并发原理"><a href="#并发原理" class="headerlink" title="并发原理"></a>并发原理</h2><p>Java 提供的基础容器都是线程不安全的，如果并发条件下多个线程同时对一个容器中的数据进行操作，可能会导致各种意想不到的错误。</p>
<p>因此 Java 又提供了一些并发容器在多线程情况下使用，这些并发容器都位于 java.util.concurrent 包内，使用时需要进行导入。</p>
<hr>
<h2 id="List-接口"><a href="#List-接口" class="headerlink" title="List 接口"></a>List 接口</h2><h3 id="Vector-类（已过时）"><a href="#Vector-类（已过时）" class="headerlink" title="Vector 类（已过时）"></a><del>Vector 类（已过时）</del></h3><p>【数组序列】和 ArrayList 类类似，实现了 List 接口。内部使用 Object 数组存储。</p>
<p>Vector 类内部所有方法都是同步(synchronized) 的，因此线程安全。但高并发场景下非常容易阻塞，性能很差。</p>
<h3 id="CopyOnWriteArrayList-类"><a href="#CopyOnWriteArrayList-类" class="headerlink" title="CopyOnWriteArrayList 类"></a>CopyOnWriteArrayList 类</h3><p>【数组序列】和 ArrayList 类类似，实现了 List 接口。内部使用 Object 数组存储。</p>
<p>CopyOnWriteArrayList 对读取操作不上锁，对写入操作上锁。写入操作通过创建数组的副本来实现，修改的内容写入副本后再替换原来的数据。</p>
<p>不允许同时写，但读操作和写操作不冲突，在多读的场合性能非常好。</p>
<h3 id="ConcurrentLinkedQueue-类"><a href="#ConcurrentLinkedQueue-类" class="headerlink" title="ConcurrentLinkedQueue 类"></a>ConcurrentLinkedQueue 类</h3><p>【链表序列】和 LinkedList 类类似，实现了 List 以及 Deque 接口。内部使用双向链表存储。</p>
<p>ConcurrentLinkedQueue 类非阻塞，通过 CAS 算法实现线程安全，尝试更新数据时会对数据进行比对。高并发场景下如果加锁的代价很高，可以达到很好的性能。</p>
<hr>
<h2 id="BlockingQueue-接口"><a href="#BlockingQueue-接口" class="headerlink" title="BlockingQueue 接口"></a>BlockingQueue 接口</h2><p>【阻塞队列】被广泛使用在“生产者-消费者”问题中，其原因是 BlockingQueue 提供了可阻塞的插入和移除的方法。当队列容器已满，生产者线程会被阻塞，直到队列未满；当队列容器为空时，消费者线程会被阻塞，直至队列非空时为止。</p>
<h3 id="ArrayBlockingQueue-类"><a href="#ArrayBlockingQueue-类" class="headerlink" title="ArrayBlockingQueue 类"></a>ArrayBlockingQueue 类</h3><p>有界队列实现类，底层采用数组来实现。ArrayBlockingQueue 一旦创建，容量不能改变。其并发控制采用可重入锁来控制，不管是插入操作还是读取操作，都需要获取到锁才能进行操作。</p>
<h3 id="LinkedBlockingQueue-类"><a href="#LinkedBlockingQueue-类" class="headerlink" title="LinkedBlockingQueue 类"></a>LinkedBlockingQueue 类</h3><h3 id="PriorityBlockingQueue-类"><a href="#PriorityBlockingQueue-类" class="headerlink" title="PriorityBlockingQueue 类"></a>PriorityBlockingQueue 类</h3><hr>
<h2 id="Map-接口"><a href="#Map-接口" class="headerlink" title="Map 接口"></a>Map 接口</h2><h3 id="HashTable-类（已过时）"><a href="#HashTable-类（已过时）" class="headerlink" title="HashTable 类（已过时）"></a><del>HashTable 类（已过时）</del></h3><p>【哈希表】和 HashMap 类类似，实现了 Map 接口。 </p>
<p>HashTable 类内部所有方法都是同步(synchronized) 的，因此线程安全。但由于整个哈希存储区域共享一把锁，高并发场景下非常容易阻塞，性能很差。</p>
<h3 id="ConcurrentHashMap-类"><a href="#ConcurrentHashMap-类" class="headerlink" title="ConcurrentHashMap 类"></a>ConcurrentHashMap 类</h3><p>【哈希表】和 HashMap 类类似，实现了 Map 接口。 </p>
<p>并发控制使用 synchronized 和 CAS 来操作，采取分段锁机制。synchronized 对哈希存储区域的每个 key 分别上锁，只锁定当前链表或红黑二叉树的首节点，这样只要不发生哈希冲突就不会产生并发，效率大大提升。</p>
<p>JDK 1.7 中， ConcurrentHashMap 类中包含静态内部类 Segment，继承于 ReentrantLock 类用来充当锁的角色，每个 Segment 对象守护若干个保存键值对的链表，共同构成 ConcurrentHashMap 实例中的数组。在 HashEntry 类中，key，hash 和 next 域都被声明为 final 型，value 域被声明为 volatile 型。由于 HashEntry 的 next 域为 final 型，所以新节点只能在链表的表头处插入。 </p>
<p>JDK 1.8 中放弃了 Segment 臃肿的设计，取而代之的是采用Node + CAS + Synchronized来保证并发安全进行实现。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="keyword">final</span> <span class="keyword">class</span> <span class="title class_">HashEntry</span>&lt;K,V&gt; &#123;</span><br><span class="line">       <span class="keyword">final</span> K key;                       <span class="comment">// 声明 key 为 final 型</span></span><br><span class="line">       <span class="keyword">final</span> <span class="type">int</span> hash;                   <span class="comment">// 声明 hash 值为 final 型</span></span><br><span class="line">       <span class="keyword">volatile</span> V value;                 <span class="comment">// 声明 value 为 volatile 型</span></span><br><span class="line">       <span class="keyword">final</span> HashEntry&lt;K,V&gt; next;      <span class="comment">// 声明 next 为 final 型</span></span><br><span class="line">  </span><br><span class="line">       HashEntry(K key, <span class="type">int</span> hash, HashEntry&lt;K,V&gt; next, V value) &#123;</span><br><span class="line">           <span class="built_in">this</span>.key = key;</span><br><span class="line">           <span class="built_in">this</span>.hash = hash;</span><br><span class="line">           <span class="built_in">this</span>.next = next;</span><br><span class="line">           <span class="built_in">this</span>.value = value;</span><br><span class="line">       &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<hr>
<h2 id="普通容器转换"><a href="#普通容器转换" class="headerlink" title="普通容器转换"></a>普通容器转换</h2><h3 id="synchronizedList-方法"><a href="#synchronizedList-方法" class="headerlink" title="synchronizedList 方法"></a>synchronizedList 方法</h3><p>如果遇到多个线程操作同一个容器的场景，可以通过 Collections 工具类中的 synchronizedList 方法将其转换成线程安全的容器。</p>
<p>方法会被 synchronized 关键字重定义。</p>
 
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